In an automotive air conditioning compressor, when the rotation of the compressor is stopped by locking of a rotating member, the connection between a driving source and the compressor should be quickly disengaged to prevent damage to the driving parts of the automobile. Such disengagement is particularly desirable where the compressor and other auxillary equipment, e.g., an alternator, power steering, are coupled to the engine output through a single power transmission belt to ensure that operation of the other equipment remains unaffected by the compressor malfunction.
Various rotation detecting devices have been proposed which detect compressor locks by sensing changes in the rotational speed of the compressor and interrupting the driving force to the compressor when the rotational rate falls below a predetermined reference rate. One such rotation detecting device comprises a magnetic flux changing portion, which varies the magnetic flux density formed by a magnetic pickup in accordance with the rotation of a drive shaft, and a magnetic detecting device, which detects the change in flux density. The construction of those prior art devices is, however, very complicated, and may also be difficult to complete. Further, such devices suffer from reliability problems. For example, if a magnet is used as the flux changing portion which has a relatively large magnetic flux, it may absorb iron grains from the interior of the compressor, resulting in unreliable results. The reliability of the device may also be adversely affected by temperature changes in the interior of the compressor as some magnets are temperature sensitive and will lose their magnetic properties if the temperature increases above, or decreases below, a certain level.